Production of cycloalkyl nitrites



Match 29, 1949. UM r AL 2,465,984

PRODUCTION OF CYCLOALKYL NITRITES Filed May 21, 1946 ORNEY Patented Mer.29, 1949 2,465,984 PRODUCTION OF CYCLOALKYL NI1BI1ES Thomas F. Donnant,

Coe, Long Inglewood, pany of Cali Beach, an

poration of California Application May 21, 1946. Serin! No. 671,394

14 Claims. c1. 260-466) This invention relates to a method for theproduction of esters or nitrous acid and reaction products thereof andrelates particulafly to the esterification of saturated hydrocarbons,especially of cycloalkyl hydrocarbons.

Esters of nitrous acid have been prepared in the past by the usualesterification reaction between an alcohol and an acid to form the esterand water. In the case of nitrous acid esters however, theesteriflcation process is complicated by the instability of nitrous acidas well as the reactivity of the esters formed. Possibly this expiainsthe fact that there is comparatively iittie information available onthis esterification. The few processes described in the prior artgenerally involvethe reaction of an alcohol with nascent nitrous acidformed in situ by acidification of a nitrite sait. Thus the method Vdescribed on page 108 of Organic Syntheses,

0011., vol. 2, published by John Wiiey and Sons Inc. of New York city,for the preparation of n-butyl nitrite involves the gradual addition ofsulfuric acid and n-butyl alcohoi to a sodium nitrite solution in thecold. As far as applicants are aware th preparation of esters oi nitrousacid by direct esterification of hydrocarbons has L never been carriedout.

Applicants have discovered -that esters of nitrous acid, or organicnitrites, as they may also be termed, may be prepared by reactingalcohois, especially cycloalkyl alcohols, with nitrogen tetroxide in theliquid phase, at atmospheric temperatures or beiow, in the absence ofcataiysts.

Applicants have also discovered another entirely unexpected reaction, i.e., that the same esters of nitrous acid may aise be prepared byreacting the same reagent, nitrogen tetroxide, with a difierentmaterial, nameiy the hydrocarbon rather than the alcohol. It is mostunusuai that the same reagent wiil react with two different materials toform the same product.

It has been discovered that if saturated hydrocarbons are maintained atan elevated temperature in the liquid phase, and contacted for areIatiVeIy short contact time with a minor proportion of nitrogentetroxide, good yields of the organic nitrites may be obtained. In thisproces it is prterable that the nitrogen tetroxide be dissolved in thehydrocarbon, and that the addition of water or nitric acid be avoided,since the presence of any substantial amount of water or nitric acid inthe reaction zone tends to reduce the production of the nitrite andresults in the formation of nitro compounds and x1- Los Angeiea.Clarence 8. d Edward C. Attan, Jr., Calit., assignors to Union 011Comfomia, Los Angeles, Caiif.. a cordation products. Continuonsconcurrent flow actant. and substantiafly nitrogen te troxide isutilized for the esterificaof the hydrocarbon and the nitrogen tetroxideis also preferred.

The main reaction involved in the process of this invention is believedto be as follows:

(1) Hi H H:C C H1C C\ N104 l e l i E HNOI H: H: H:

C C H] H! Ni en C clohexane Cyclohexyl- Nitrous tetode y nitrite acidSince nitrous acid is very unstabie, the principal products other thanthe nitrite are nitric-oxid and water rather than nitrous acid; thusf eoverall reaction is believed to be:

( 3 l 4 eHu CaEnONO 2N0 21150 Nitr en Cycio Oyciohexyi Nitric Watertetrox de hexane nitrite oxide The nitric oxide of Equation 2 mai bereadily oxidized to nitrogen tetroxide by t e following reaction:

(3) 2N0 o. N,O.

Nitric Oxygen Nitrogen oxide (ai tetroxide Thus the by-product nitricoxide n1ay be very readily reconverted to the nitrogen tetroxide reallof the nitrogen cf tion.

It has aiso been discovered that the organic nitrites may be readilyconverted to additional valuable products. For example, the cycioalklnitrites have been found to be very readily oxidized to dibasic acids ofunusuaily high purity,

This provides an improved process for the production of such dibasicacids, namely, converting the cyclalkanes to the cycioaikyi nitrites,and oxidizing the latter to the dibasic acids." In the case ofcyclohexane for example, the cyclohexyl nitrite prepared by equations 1or 2 above may be oxidized to adipic acid. The overall exidationreaction apparentiy is as foliows, where nitric acid is used as theoxidizing agent:

3 uHI1ONO 7EN03 Cyclohexyl Nitric nitrite acid 3COOH(CH:) COOH 511,0i0NO Adipic acid Water Nitric oxide The nitric oxide by-product may bereconverted .tored heat, and Water and nitric to the nitrogen tetroxidereactant employed in Equation 1 or 2 by oxidation according to Equation3 above. It may also be converted to the nitric acid reactant employedin Equation 4 by the following oxidation:

(B) 4NO 302 21120 e 4HNO:

Nitric .Oxygen Water Nitric oxide acid Thus the overall reaction forproduction of adipic acid by Reactions 2, 3, 4 and 5 is simply:

() 2CqII]: 501 2H20 Cyclo- Oxygen Water hexane It has also beendiscovered that nitrogen tetroxide is much more soluble in hydrocarbonsthan are the other nitrogen oxides such as nitrous oxide or nitric oxideor other gases such as nitragen itself. This makes possible a furtherimprovement in the above process for preparing organic nitrites, namelypreparing the desired solutlon of nitrogen tetroxide in the feedhydrocarbon by selectively extracting the nitrogen tetroxide from agaseous mixture containing the tetroxide in admixture with less solublegases such as other nitrogen oxides, nitrogen, ammonia, air and watervapor. Such gaseous mixtures are ecc- 2CnHxa04 Adipic acid nomicallyprepared by the oxidation of ammonia. This oxidation is preferablycarried out in two steps, the main reaction in the first step being asfollows:

Ammoma Nitric Water oxide The second step is the oxidation of the nitricoxide, after separating it from the water, to obtain the tetroxide,according to Equation 3. The resulting gaseous mixture, containingnitrogen tetroxide in admixture with impurities such as other nitrogenoxides, nitrogen, air and the like, is then contacted with thehydrocarbon feed, which selectively dissolves the nitrogen tetroxide toform the reaction mixture.

Thus the primary process of this invention is the esterification ofhydrocarbons with nitrogen tetroxide to form the organic nitrites, andthis process may be elaborated by the use of the feed preparationprocess involving the selective extraction of the nitrogen tetroxide bythe hydrocarbon feed, and by the subsequent conversion of the cycloalkylnitrites to the dibasic acids by oxidation.

The invention will become more clear by reference to the attacheddrawing, in which cyclohexane is indicated as the hydrocarbon feed,although other hydrocarbons may also be employed, as described below.Referring to the drawing, air or oxygen, and ammonia are introducedthrough limes I and 2 and valves 3 and 4 lnto ammonia oxidation zone 5in which they are contacted with a catalyst such as platinum gauzeheated thus converted to a mixture of oxide as indicated in Equation 6:aThe resulting product is passed through line 8 and cooling means 1 intdehydration zone 8 in which the water is removed by condensation and/orother dehydration and withdrawn through line 9, while the remaining gas,largely nitrogen and nitric oxide, is removed through line in and passedto oxidation zon II. In this zone it is contacted with air introducedthrough line [2, whereby the nitric oxide is converted to nitrogentetroxide in accordance with Equation 3. The resulting gas, principallynitrogen and nitrogen tetroxlde is removed through line l3, and theOxygen nitrogen tetroxide ls selectively dissolved in cyclehexane inabsorption system Il. This system may be in the form of a countercurrentabsorber in which liquid hydrocarbon feed is introduced in the upperpart of the absorber, and the nitragen tetroxide containing gas 15introduced into the lower part of the absorber. The non-absorbed gas 15withdrawn through line I! and discarded, or if sufiiciently rich innitric oxide, may be returned to oxidation zone II. The solution ofnitrogen tetroxide in cyclohexane is withdrawn through line [8 and sentto reactiou Zone I, in which the cyclohexane is converted to cyclohexylnitrite in accordance with Equation 1 or 2. The resulting product iswithdrawn through line 20 and separated in separation zone 2l to obtaina gas fraction largely nitric oxide, which may be withdrawu through line22 and returned to oxidation zone 1 I, a small amount of an aqueousphase which may be withdrawn through line 23, and an 011 phase which maybe withdrawn through line 24 and sent to recovery system 25. In therecovery system 25, which may be merely a fractional distillation zoneor any other type of separation system, the unreacted cyclohexane may beseparated, withdrawn through'line 28 and returned to absorption systeml4; the cyclohexyl nitrite may be recovered and withdrawn through line21, and by-products including nitrocyclchexanemay be withdrawn throughline 28. The cyclohexyl nitrite may then be converted to adipic acid byintroducing it into oxidation zone 29 where it is contacted with nitricacid introduced through line 30 and converted to adipic acid inaccordance with Equation 4. The resulting product may be transferred torecovery zone Il through line 32. A gas fraction, which is pri marilynitric oxide may be withdrawn through line 33 and retumed 1:o oxidationzone H, and the remainlng material may be treated for recovery of theadipic acid which is withdrawn through line 34, and by-products such assmall amounts of other dibasic acids, withdrawn through line 35.

It must be emphasized that the process as shown in the drawing is merelyillustrative of the invention, and the invention is not llmited thereto,since other cycloalkanes, oxidizers,etc., may be used, as describedbelow. A more complete description of the conditions et operation ofeach stage of the processwill flrst be presented bowever.

The oxidation of the ammonia in zone 5 ls known, being described forexample, in an article by Guy B. Taylor, entitled Oxidation o: ammonia,found in the Journal 01 Industrlaland Engineering Chemistry, vol. 19(1927), page 1250. It is there shown that various catalytic agents maybe employed and that the reaction is substantially complete, VTemperatures above 700 0. are used. An excess of oxygen is desirable,the relative amounts of air and ammonia being generally about air to 10%ammonia. Thls is about 50% more oxygen than required by Equa tion 7.

In the process described in the Taylor article and the other prior art,the mixture of nitllc oxide and water resulting from the above ammoniaoxidation ls converted to nitrlc acid rather than to nitrogen tetroxide,i. e., the reaction oi Equation 5 rather than that of Equation 3 iscarried out. Both reactions occur very readfly at atmospherictemperatures or slightly elevated temperatures, but for the productionof nitrlc acid, water is essential, whereas for production of nitrogentetroxide it -is most desirable that the presence of any appreciablequantity of water be avoided. Accordingly, the water is removed. Thedehydraticn in zone 8 may be merely a cooiing or combination of coolingand compressing suflicient to reduce the water content of the gas to aconcentration lower than that of the nitric oxide and preferably lessthan of the concentration of the nitric oxide. Driers such as calciumchloride, silica gel or the like may also be employed if desired.

The oxidation of the nitric oxide in zone Il may be carried out atatmospheric pressure.but

is preferably carried out under moderate pressures such as about 50 to500 pounds per sq. in. Temperatures between about 0 C. and about 100 C.may be used, preferably about to 50 C. An excess of air up to 2 or 3times theoretical should be used. Contact times need not be over aboutten minutes and may be as short as one-half second or less.

As an example of the conversion of ammonia t0 nitrogen tetroxide by the2-stage process described above air and ammonia in the proportions ofabout 90% air to 10% ammonia are preheated to a temperature of about 700C. and the mixture is passed at substantially atmospheric pressurethrough a. platinum gauze heated to red heat. The resulting mixturecontains about 9% of nitric oxide together with about 7% of excessoxygen, 13% Water, and less than 1% unconverted ammonia, the remainderbeing oxygen. This mixture is cooled and compressed to a temperature inthe neighborhood et 100 F. and a pressure of about 100 pounds per sq.inch gage, under which conditions the bulk of the water settles out asan aqueous nitric-acid contaiuing condensate, which is withdrawn. Theresulting partially dehydrated gas is substantially completelydehydrated by passing it through a silice. gel drier bed. The nitricoxide in the dehydrated mixture is then converted to nitrogen tetroxideby adding air to the mixture in such a. proportion that there is about a50% excess of oxygen over that required by Equation 3. At 100 F. and 100pounds per sq. in. gage the conversion of 1e nitric oxide to nitrogentetroxide is comple. in about 10 to 15 seconds, giving as a product agas containng about 6% of nitrogen tetroxide (inciuding dioxide), about4% of oxygen, and less than 1% of nitric oxide, ammonia, and water, theremainder being nitrogen.

The separation of the nitrogen tetroxide from the above gascons mixtureis preferably carried out by selective absorption in the hydrocarbonfeed in zone 14 as indicated in the drawing, since this method ofseparaticn automatically accomplishes the also desired effect ofdissoiving the nitrogen tetroxide in the hydrocarbon feed stock.

The absorption in zone i4 is preferably carried out at temperaturesbelow about C. down to about 10 0., or below, and superatmosphericpressures such as about pounds to 500 pounds per sq. in. gage. Asufiicient proportion of hydrocarbon to nitrogen tetroxidecontaining gasis employed to remove the bulk of \the nitrogen tetroxide from themixture. A snall amount of the other gases may also be tolerated. Theequipment employed may be a conventional absorber such as employed fornatural gasoiine production or the like, i. e., a packed or bubble captype tower. The gas stream is injected in the lower part of the towerand travels upward countercurrently to a down flowing stream of thehydrocarbon, which is introduced near the top of the tower. Theunabsorbed gas is removed from the top of the tower. and may bediscarded or further treated for recovery of desired constituents. Ifthere is sufl1- cient nitric oxide in this unabsorbed gas it may bereturned to dehydration zone 8 or oxidation zone i l; or if there issufficient ammonia in this stream, it may be recycled to oxidation zone5. Conventional methods of separation and concentration of ammonia ornitric oxide may also be employed prior to the recycling as indicated.The solution of nitrogen tetroxide in the hydrocarbon feed is withdrawnfrom the bottom of the tower in the above operation.

As an example of the above absorption process for separaticn of thenitrogen tetroxide, the mixture previously described, containing about6% of nitrogen tetroxide, is cooled to a temperature of about 10 C.without substantially reducing the pressure, and is passedcountercurrently to a stream cf cyclchexane in a packed tower. Theunabscrbed gas withdrawn from the top of the tower contains about 1% ofnitrogen tetroxide or less, and the bulk of the nitric oxide, ammonia.oxygen, and nitrogen, while the cyclohexane solution withdrawn from thebottom of the tower contains about 10% by weight of nitrogen tetroxide.Similar absorption processes are carried out with the other hydrocarbonfeed stocks of this invention under the same conditions or otherconditions within the above ranges, to obtain suitable separaticns ofnitrogen tetroxide and to prepare suitahle solutions for oarrying outthe esterification process of this invention whereby the hydrocarbonfeed stocks are reacted to form the nitrites.

The conditions which we have found favorable for the formation of thenitrites from the corresponding cycloalkanes in reaction zone I9 40 aresuperatmospheric temperatures below the critical temperature of the feedstock, i. e., temperatures between about 25 C. and 400 C., proferablybetween about 100 C. and 250 C. superatmospheric pressures up to about2000. pounds per sq. in. or higher may be used, preferably pressuresbetween about pounds and about 300 pounds per sq. in. gage. The pressureshould be suficiently high to insure a substantially liquid phasereaction. By this term it is meant that the hydrocarbon is substantiallycompletely in the liquid phase, and some nitrogen tetroxide is dissolvedtherein. All of the nitrogen tetroxide need not be dissolved in thehydrocarbon, a1- though it is desirable that great intimacy et contactbetween the hydrocarbon and any gasoous nitrogen tetroxide be obtained.It has been found desirable to dissolve the nitrogen tetroxide in anexcess of the hydrocarbon at a relatively low temperature, such as about40 C. or below and introduce this solution at this low temporaturecontinuously into the reactor. By this mode of operation continuonsconcurrent fiow is obtained. and the nitrogen tetroxide is liberated asthe solution is warmed to reaction temporature, butis liberated inextremely small bubbles,

which provide the desired intimacy of contact. This type et operation isbelieved to be of value in any situation in which great intimacy ofcontact is desired between a liquid and a gas, namel dissolving the gasin the liquid at a lower temperature, and allowing the gas to beliberated in the reactor as the solution is warmed to the reactiontemperature.

It has also been found desirable to employ a 5 very short contacttime, 1. e., preferably less than action.

about 15 minutes, and no longer than 15 necessary to form the desirednitrite, so as to minimize decomposition and isomerization of thenitrite-t the nitro-hydrocarbon. In the continuous process anyaccumulation of an aqueous phase in the reactor should also be avoided.As observed from Equation 1, nitrous acid may be formed in the reaction,and nitric acid is formed by its decomposition. Water and nitric oxideare also 1'ormed, as indicated in Equation 2, the water in reiativelysmall amounts. If allowed to accumulate in the reactor; this water oraqueous phase may accelerate side reactions such as hydrolysis,

nitration, and oxidation resulting in the production of nitro compoundsand dibasic acids. Therefore the conditions in the reactor should besuch as to prevent accumulation of aqueous phase there, e. 3., the flowthrough the reactor should .-be sufficiently rapid to prevent retentionof the aqueous phase. Obviously, the desired control of reaction timeand velocity of flow may be obtained by appropriate reactor design as tolength and diameter. Downfiow may also advantageously be employed. Theproportion of nitrogen tetroxide to cycloalkane should be relativelylow, such as between about 1% and 25%, and preferably in the region ofabout 5% t0 15% by weight. This permits substantially completeutilization 01 the nitrogen tetroxide in each pass through the reactor.Conditions of temperature and contact time should also be adjusted toreduce the nitrogen tetroxide only to nitric oxide, rather than tonitrogen or nitrous oxide, since the latter two are rather diificult toreoxidize to nitrogen tetroxide, whereas the reoxidation of the nitricoxide to nitrogen tetroxide occurs .substantially in the liquid phase.

spontaneously at atmospheric temperature in contact with air.

The entire reaction product withdrawn from zone I! may be transferreddirectly to oxidation zone 29 as described below, or may be :flrstseparated in zone 2l, merely by stratification, with or without cooling,to obtain a gas phase and a liquid phase. The gaseous fraction obtainedfrom separator 2|, which is largely nitric .oxide, may be merelyrecycled to zone II, where it is contactd with air and reused in the re-It may also be purified of the bulk of the contaminating water vapor byrecycling it to cooler 1 or dehydration zone 8. It may also be recycledto absorption zone I4 if it contains an appreciable quantity of nitrogentetroxide, thereby purifying it of water, nitrogen, nitric and nitrousoxides and the like.

The liquid reaction product may be transierred directly to oxidationzone 29 as described below, or may be first separated into an oil phaseand an aqueous phase. The aquedus phase may be discarded or treated byknown methods to recover nitrogen oxides or nitric acid thereirom. The011 phase may be transfard directly to oxidation zone 29, or may befirst treated to obtain an unreacted hydrocarbon fraction which isrecycled entirely or in part to the absorption or reaetion step. Theremaining material, after part or all of the unreacted hydrocarbon hasbeen removed, may be transferred directly to oXidatien zone 29, or mayflrst be treated to separate the desired nitrite and other products ofside of separation may also be used. Prier te disfiliation it isdesirable to remove residual traces of nitrogen oxides and water fromthe 011 phase. This may be done by passing air over the vigorouslystirred 011 phase to remove the gases, and by dehydrating with anhydroussodium sulfate. Other conventional methods of purification may also beemployed.

As a specific example of the esterification process of this invention asolution containing 9.1% by weight of nitrogen tetroxide in cyclohexaneis prepared as above by two-stage oxidatien cf ammonia to nitrogentetroxide, foliowed by absorption of the latter in cyclohexane. Thissolution 15 1'ed at an average rate of 100 ml. per minute through aspray nozzle at the bottom of a 6 ft. vertical reactor of H; inch I. D.stainless tubing. packed w th 4 mm. glass Raschig rings. The reactor isequipped with heating elements, so that the Ieed, though introduced at atemperature of about 20 C. is heated to a temperature of about 100 C.within approximately the lower one foot et reactor, and is furtherheated in the remainder of the reactor to a maximum temperature of about152 C., the average temperature through the upper of the reactor, whichis referred to herein as the reaction tem- .perature, being about 135 C.The pressure on the reactor is approximately 100 pounds per sq. in.-gage, so that the cyclohexane is maintained Under these conditions theaverage contact time at the reaction temperature et 135 C. is about 3minutes, and the 'nitrogen tetroxide is substantialiy completelyconsumed forming a gas which is largely nitric oxide. A very smallamount of aqueous phase, largely nitric acid, is formed in the reactor,and is carried through the reactor with the oil phase.

40 The reaction products are withdrawn from the top of the reactor, andcooled to about room temperature. The oil phase is separated from thegas and aqueous phases, and subjected to distillation to rec0verunreacted cyclohexane and cyclohexyl nitrite thereirom. The remainder isfurther distilled to recover cyclohexanoi and nitrocyclohexane assuccessive overhead distillate tractions, leaving a residual bottomsfraction belieVed to contain polymers and poly-substituted cyclohexanessuch as nitrocyclohexanols, dinitro. cyclohexanes,nitroso-nitrocyclohexanes, nitritonitrocyclohexanes, and the like.

Under the above conditions, about 8% of the cyclohexane reacts, thisbeing neariy the theoretical amount according to Equation 1, and theremainder of the cyclohexane is suitable for recycling. Of thecycIohexane-ree reaction products, about 27% is cyclohexyl nitrite. Theremainder consists of about 6% cyclohexanol, 49%

test is described in the Industrial and Engineering Chemistry,Analytical edition, vol. 13 (1941), page 555.

Under other conditions within the ranges o f temperature, contact time,etc., defined above. cyclohexane is also esterified to obtainsubstantial yields of the nitrite, as well as other reaction productsdescribed above. Similarly. under these conditions other cycloalkanehydrocarbons are esterifled, such as cyclopentane, methylcyclopentane,dimethylcyclopentane, ethylcyclopentane,

methylcyclohcxane, and the like, and the corresponding cycloalkylnitrites are obtained. Upon oxidation of these nitrites in zone 29howevcr, as indicated in the drawing, it is clear that adipic acid willnot be the only dibasic acid obtained. In the case of cyclopentylnitrite substantiaily pure giutaric acid is produced, and in the case ofthe nitrites of the alkylated cycloalkanes, mixtures of adipic,glutaric, succinic and other acids are formed.

The above cycloalkanes need not be pure, since the esterification willalso take'place with mixtures of difierent cycloalkanes, and withmixtures of cycloalkanes with other hydrocarbons such as minorproportions et paraffins or aromatic hydrocarbons of similar boiiingpoint. A mixture containing about 80% of cyclohexane and about 20% ofisparafflnszof similar boiling point, for example, may be regdilyobtalndby' fractionation of petroleum, "andz is an excellent stock.

Although the above description has been confined largeiy to thecycloalkanes, which are the preferred stocks of this invention, it hasbeen found that other saturated hydrocarbons, such as the normalparaflins, pentane, hexane and the like or isoparafiins such asisopentane, isooctane and the like will also form nitrites under theabove conditions, and such processes are also within the scope of thisinvention. As pointed out above pure hydrocarbon feed stocks need not beused, although they are preferred; and pure nitrogen tetroxide also isnot necessary although it is preferred. Minor proportions of relativelyinert gases such as nitric oxide, nitrogen, or nitrous oxide and ammoniamay also be present.

As a speciflc example of the esteriflcaticn of acyclic alkanes, 20 g. ofnitrogen tetroxide is dissolved in 180 g. of isooctane and the solutionis charged to a bomb, pressured with nitrogen to 100 lb. gage, andheated at about 70 C. for about 26 hours. The nitrogen tetroxide issubstantiaily entirely reduced to nitric oxide, and on topping ofi theunreacted isooctane and vacuum distilling the residue, a distillatefraction of about 2 ml;, having a boiling point of about 59.6 C. at 4.4mm. and a refractive index at 20 C. of about 1.4281, and giving a verypositive charactcristic test for nitrite, is obtained.

Normal paraffins may also be esterified as above, and valuable productsmay be prepared. For example, by esterificaticn of C14 to Cm normalparafins, the corresponding nitrites are obtained, and these are readilyhydrolyzed to form the corresponding alcohols, which in turn aresulfated with sulfuric acid to obtain excellent detergents.

Retuming now to the cycloalkyl nitrites prepared.as above, these arevery susceptible to oxidation to dibasic acids. For example, liquidcyclohexyl nitrite retained in a glass container on the laboratory shelfexposed to difiused light apparently undergoes auto-oxidaticn andreduction, and over a period of about a week, a substantiai formation ofcrystals occur,. which crystals. are

round to be adipic acid. In general, the cycloalkyl.

nitrites are readily oxidized to dibasic acids at organic persalts suchas sodium perchlorate, po-

tassium permanganate, sodium dichromit and the like may also be used. Anexcess of oxidizing agent should generally be employed. Reactiontemperatures shouid be between about 25 C. and

and 250 C.; pressures may be between about 0 pounds and 1000 pounds persq. in. gage; and contact times just long enough to obtain the desiredconversion may be used. The reaction la preierably carried out in theliquid phase. Meahs should be provided formaintaining good contactbetween the cycloalkyl nitrite and the 0X1- dizing agent.

When nitric acid is employed as the oxidizfilng agent, temperaturesabove about 50 C. are preferred since these higher temperatures favorthe oxidation reaction over the competing nitration reaction.Excessivcly long reaction times should be avoided, since these tend todegrade the purity of the dibasic acid product. Nitric acid of thestrength greater than about by weight 18 prefcrred, aithcugh lowerconcentrations may be employed at the higher temperatures within theabove range and at higher contact times. Atmospheric pressure issuitable although super atmospheric pressures may also be employed. Itis desirable to expel the nitrogen oxides from the reaction mixture asrapidly aspossible. and for this purpose an inert gas such as nitrogenmay be used as a stripping gas if desired. Because of the highreactivity of the cycioalkyl nitrites, the presence of an inert diluantin the reaction mixture 1s desirable.

As a speciflc example of the above oxidati0n, using nitric acid, to 50ml. of boiling nitric acid (70%) at atmospheric pressure over a refluxcondenser is added duririg 15 minutes, 50 ml. of a 10% solution byvolume of cyclohexyl nitrite in cyclohexane. Oxides of nitrogen arerapidly evolved. The mixture is refluxed for five minutes after theaddition of all the cyclohexyl nitritecyclohexane solution, and is thencooled, causing the separaticn of crystals of adipic acid from theaqueous phase. The cyclohexane phase la separated from the aqueousphase, and the latter is evaporated to dryness to give a solid whiteproduct consisting of adipic acid of over 98% purity. This is asurprising degree of purity. since adipic acid produced by the oxidationof cyclohexancl, cyclohexan0ne, or cyclohexane with nitric acid, by themethods of the prior art.

does not exceed about 60 to 70% purity. The cyclohexane phase in theabove speciflc example is found to be entireiy free of cyclohexylnitrite.

The above example shows that the cyclohexyl nitrite is oxidizedselectively to substantially pure adipic acid, without appreciabiereaction of the cyclohexane. ilarly selectively oxidized in the presencecf cycloaikanes, and are also seiectively oxidized in the presence ofnitrocycloalkanes. This permits a very efficient combination of theesteriflcaflon reaction with the oxidation reaction, since the entirereaction product from the esteriflcation may be reacted with anoxidizing agent and the nitrite thereby selectively oxidized to thedibasic acid. As an example of this type of oxidation, the aboveoxidation with boiling 70% nitrlc acid is carried out in the samemanner. employing in place of .the above cyclohexane-cyclohexyl n1-Other cycloalkyl nitrites are sim- -clohexane, and other reactionpmducts. It is iound that under these conditions, the cyclohexane andnitrocyclohexane are substantially unaflected, and substantially all ofthe cyclohexyl nitrite, as well as a portion of the other esteriflcationreaction products, are converted to adipic acid. Similar results areob'tained by employing the entire esterification reactin product,without separation of gas or aqueous phases; or by employing an oilphase which has been Ireed irom part or all of its unreacted cyclohexanecontent.

'Ihus a high quality adipic acid may be produced by subjecting theproduct of the esteriflcation reaction as described above directly tothe oxidation reaction as described above; or between the esteriflcati0nand the oxidation there may be a separation of the gas phase or of theliquid phase or ci. 8. portion 01. the unreacted hydrocarbon feed stock,or of any or all of these. Also if desired other products such asnitrocycloalkanes or ccloalkyl alcohols or the like may be separatedbefore the oxidation.

Nitrogen tetroxide is also an excellent oxidizing agent for theconversion 01. the cycloalkyl nitrites to disbasic acids as above. Forexample, a solution of about of nitrogen tetroxide and 10% cf cyclohexylnitrite in cyclohexane is decolorized at room temperature in a matter ofa few hours, 1. e., all 01 the nitrogen tetroxide is reduced to nitrlcoxide. with the production of adipic acid, which settles as acrystalline solid from the solution. Nitrogen tetroxide may also beemployed under other conditions within the above limits, to oxidizecyclohexyl nitrite or other cycloalkyl nitrites to dibasic .acids. Theuse of nitrogen tetroxide iurthermore, permlts the simultaneousesterification of cyclohexane and oxidation 01. the nitrite ester toadipic acid. The latter reaction proceeds at a much faster rate, so thatwhen a solution 01 about 10% of nitrogen tetroxide in an excess ofcyclohexane is allowed to react at room temperature in a clear glasscontainer in the laboratory, crystals of adipic acid appear within abouttwo days, and within about two weeks, the solution is completelydecolorized. Substantially no cyclohexyl nitrite or nitrocyclohexane isobserved in the product, which is substantially pure adipic acid andunreacted cyclohexane, the nitrogen tetroxide being reduced to nitrlcoxide. This is explainable on the basis that the cyclohexyl nitrlte isthe primary reaction product. and this is oxidized to adipic acid at afaster rate than its rate of formation, so that the concentration of thenitrite at any given time is almost negligible. Ii greater yields of thenitrite are desired, without further oxidation to adipic acid, acontinuous system should be employed wherein an excess oi liquidcyclohexane containing dissolved nitrogen tetroxide is passedcontinuoualy and concurrently through a tube at the desired temperature.Temperatures between about 50 C. and 200 C. are preferred. If it isdesired to oxidize the nitrite to dibasic acid directly as produced,either a batch system should be employed as described above, or aconcurrent system should be used, wherein the cyclohexane is allowed toflow countercurrently to a stream of nitrogen tetroxide.

, 12 In the oxidation of the nitrite with either nitrlc acid or nitrogentetroxide, nitrlc oxide is produced as a by-product. Since thls isreadily A reoxidized to nitrogen tetroxide by air or oxygen, it isadvantage0us to introduce oxygen or air into the reaction mixture,either together with the feed in a batch or concurrent system, or at anintermediate stage of the reaction in a countercurrent system, or at asubsequent stage of the reaction in a concurrent system. It has alsobeen discovered that nitrlc oxide has a catalytic eflect on theoxidation, so it is advantageous to introduce nitrlc oxide with thereactants. This is true not only for the oxidation reaction but also foresterlflcatlon reaction. Furthermore, other products of theesteriflcati0n reaction, such as the nitrite, and the other productsmentioned above. also exert a catalytic effect on the esterificafionreaction. Ihus the recycling of the unreacted hydrocarbon, and alsorecycllng of 8. portion of the reaction products, is desirable. Largeareas et surface contact also appear to be advantageous in both theesteriflcation and oxidation reaCtionS.

By employing, roxides as oxidizing agents, the cycloalkyl nitrites maynot only be converted to dibasic acids, but may also be converted tonitrates. As far as applicants are aware, cycloalkyl nitrates have notpreviously been prepared. It is also within the scope oi! this inventionto prepare cycloalkyl nitrates by the reaction 01 the cycloalkylnitrites of this invention with Deroxides, whether hydrogen peroxide ororganic hydroperoxides such as tertiary butl hydropr oxide, dimethylcyclopentyl hydromroxide,

methyl cyclohexyl hydroperoxlde and the like, or

other organic peroxides 'such as benzoyl peroxide and the like. In thisreaction the hydrogen peroxide is reduced to water; the organichydroperoxides are reduced to the corresponding alcohols; and thebenzoyl peroxide is reduced to benzoic acid anhydride.

In this reaction an excess of the nitrite should be employed.Atmospheric temperature and higher temperatures up to about C. aresatisfactory. The reactants are preferably employed in solution in acommon solvent so as to obtain a homogeneous reaction. 0rganic solvantsare preierred over aqueous solvents. 'Ihus the nitrites and the organicperoxides and hydroperoxides may be dissolved in hydrocarbons such asthe hydrocarbon feed stocks oi this invention or other petroleumtractions or the like. Hydrogen peroxide is preferably employed inalcohol or ether solution although it may be used in water solution. Inthe event that a water solution is employed, the reaction mixture willnot be homogeneous. and means should be Provided to assure sufllcientintimacy et contact between the peroxide and the nitrite. If desired,the hydrogen peroxide may also be obtained by the action of water ormineral acids on inorganic hydroperoxides such as barium peroxide,sodium peroxide and the like. Other oxidizing agents et the typesdescribed above may also be employed to oxidlze the nitrites to thenitrates.

As an example of the preparation of cycloalkyl nitrates, a solutioncontaining about 25% by weight of dimethylcyclopentyl hydroperoxide incyclohexane is added nadually to a solution containing about 25% ofcyclohexyl nitrite in cyclohexane, the molal proportion of hydroperoxideto nitrite being about 1 to 2. Upon sentie warming, the reaction goessubstantiay to completion, and cyclohexane, dimethylcyclopentyl alcoholand 13 l unreacted nitrite are separated from the solution bydistillation, leaving a residue which is substantially pure cyclohexylnitrate. This materla l is an excellent additive for Diesel fuels,toimprove their ignition characteristics. For this purpose they areemployed in the Diesel fuel in concentrations between about 0.1% andabout by weight. If desired, the 'preparation may be carried out byemploying the Diesel fuel itself as the diluent. Thus Diesel fuel may besubstituted for cyclohexane in the above preparation, and the resultingproduct diluted with additional Diesel fuel to the desiredconcentration. In another mode of operation, about 1% of cyclohexylnitrite may be dissolved in the Diesel fuel, and an equimolal amount ofthe hydroperoxide dissolved in Diesel fuel may be added thereto. OnSlightl Warming, the above reaction will take place. Although thereaction goes substantially to completion, this is not vital, and anexcess of unreacted nitrite or hydroperoxide, or both, may be present inthe reaction product, since all of these are efiective in improving theignition characteristics of the Diesel fuel.

The other cycloalky] nitrites and the other peroxides and otheroxidizing agents described above may also be reacted as above to obtainthe corresponding cycloalkyl nitrates, which may be utilized as above.

Many modifications of the processes of this invention may be employed.For example, the nitrogen tetroxide need not be prepared by theoxidation of ammonia as described above, but may also be prepared bydirect oxidation of nitrogen of the atmosphere by the Birkeland-Eydeprocess, or a modification thereof recently disclosed in the literature,whereby a fuel such as methane is burned in an excess of air at atemperature in the neighborhood of 4,000 F. or above to form nitricoxide, which is oxldized to the tetroxide as described above. In thismode of preparaticn, the concentration of the nitric oxide or nitrogentetroxide is quite low, and consequently it may be desirable to employother modes of purification in addition to or in place of thosedescribed above, such as for example, low temperature fractionaldistillation, charcoal adsorption, silica gel adsorption, and the like.

It is also apparent that in the course of the above esterificaticn andoxidation reactions nitric oxide may be liberated, and this is readilyoxidized to form the desired nitrogen tetroxide. Furthermore, nitricacid itself may be decomposed to form nitrogen tetroxide, which may beextracted or separated from the product by the methods above. 'Nitricoxide formed by the decomposition of nitric acid may also be oxldized tosupply the necessary nitrogen tetroxide.

As mentioned previously, catalysts may be desirable in theesterification reaction as well as the oxidation reaction. It is alsodesirable to employ a dehydrating agent in the esterificaticn reacticnzone itself, so that the water liberated in the reaction is not allowedto hydrolyze the nitrite esters. Silica gel and diatomacecus earth aresuitable dehydrating agents, which also act as catalysts. These areintroduced in suspension in the feed, or are placed in the reactor as abed. Light, especially ultraviolet light, is a catalyst for the aboveesterificaticn also, and appears to inhibit the production of nitrohydrocarbons as well as catalyze the production of the nitrite and theoxidation products thereof. Also, as previously pointed out, recyclingof the esterification reaction products is desirable because ofcatalytic effects of nitric oxide and other reaction products. Recyclingof nitro hydrocarbons has the additional desirable effect of inhibitingthe 'nitrite to form cyclohexyl alcohol and ethl thionitrite. Thethionitrites are also excellent additives for improving the ignitionquality Dieselfuels. This reacticn also takes place at atmospheric orslightly elevated temperatures, and is preferably carried out in 011solutions.as with the preparation of the nitrates above described.

As indicated above the nitrite esters of-this invention are of greatvalue because of their great reactivity to form other chemicals.However, they are also valuable in themselves as ignition promoters forDiesel fuels or jet propulsion fuels, as effective componentsofinsecticides or fungicides, and for other uses.

Although the term nitrogen tetroxide is used exclusively herein it is tobe understood that this term includes other forms which may be inequilibrium with the tetroxide such as nitrogen dioxide. As shown byRiebsomer in The reactions of nitrogen tetroxide with organic compounds,in Chemical Reviews, vol. 36, No. 2, April 1945, page 159, the structureof nitrogen tetroxide is not definitely known and undoubtedly changeswith changes in temperature or other conditions. We have found howeverthat nitrogen tetroxide is very diflerent in its reactions withhydrocarbons than is nitric acid. In the presence of appreciable amountsof water or nitric acid the organic nitrites are apparently not formed,or possibly are immediately destroyed with the production of oxidationor isomerizaticn products.

Other modifications of this invention which would be apparent to oneskilled in the art are to be included within the scope of the inventionas defined in the following claims.

We claim:

1. A process for the production of cycloalkyl nitrites which comprisesdissolving a minor proportion of nitrogen tetroxide in a cycloalkane,heating the resulting solution in the liquld phase so as to form thecorresponding cycloalkyl nitrite and separating from the Iiquid producta fraction rich in said nitrite.

2. A process for the production of a cycloalkyl nitrite which comprisesdissolving a minor proportion of nitrogen tetroxide in a cycloalkane,-

maintaining the resulting solution at an elevated temperature so as toform a liquid product containing the corresponding cycloalkyl nitriteand a gascons product containing nitric oxide, separating said nitriteand unreacted cycloalkane from said liquid product, recycling saidunreacted cycloalkane, oxldizing said gaseous product containing nitricoxide to obtain additional nitrogen tetroxide, and recycling saidnitrogen tetroxide.

3". A process for the production of cycloalkylnitrites which comprisesoxidizing ammonia so as to obtain an ammonia oxidation product con-=taining water and nitric oxide, dehydrating the said ammonia oxidationproduct to remove said water therefrom, oxidizing the dehydrated am::monia oxidation product to obtain a gaseous mix-=. turc containingnitrogen tetroxide, contacting:

said gascons mixture with a liquid cycloalkane thereby absorbing saidnitrogen tetroxide in said cycloalkane to form a solution containing aminor proportion of nitrogen tetroxide, maintaining the resultingsolution et au elevated temperature so as to form a liquid productcontaining the corresponding cycloalkyl nitrite and a gaseous productcontaining nltric oxide, separating said nitrite and unreactedcycloalkane from said liquid product, recycling said unreactedcycloalkane, and recycling said gaseous product containing nitric oxideto the stage in which the dehyclrated ammonia oxidation product isoxidized to form nitrogen tetroxide.

4. A process for the production of cycloalkyl nitrites which comprisesdissolving a-minor proportion of nitrogen tetroxide in a. cycloalkane,introducing the resulting solution having a temperature lower than about40 C. into the reaction zone, heating the solution in said reactionzone, to a reaction temperature greater than about 100 C., maintainingthe solution in the liquid phase for a suflicient contact time to formthe cycloalkyl nitrite, removing the reaction mixture from the reactionzone, and recovering said cycloalkyl nitrite theretrom.

5. A process for the production 01 cycloalkyl nitrites which comprisesdissolving a minor proportion of nitrogen tetroxide in a cycloalkane,introducing said solution having a temperature below about 40 C. into areaction zone, maintaining said solution as it passes through saidreaction Zone substantially in the liquid phase while heating it to areaction temperature above about 100 C., maintaining it at saidtemperature while reacting said nitrogen tetroxide with part of saidcycloalkane so as to form a liquid product contalning unreactedcycloalkane and cycloalkyl nitrite, separating said unreactedcycloalkane from said product and recycling it, and recovering saidcycloaikyl nitrite from the remainder of said product.

6. A process for the production of cycloalkyl nitrites which comprisesdissolving a minor proportion of nitrogen tetroxide in a cycloalkane,introducing said solution having a temperature below about 40 C. into areaction zone, maintaining said solution as it passes through saidreaction zone substantially in the liquid phase while heating it to areaction temperature above about 100 C. and maintaining it at saidtemperature while reacting said nitrogen tetroxide with part 01 saidcycloalkane so as to form a liquid product containing unreactedcycloalkane and cycloalkyl nitrite and a gaseous product comprisingnitric oxide, separating said gaseous product from said liquid product,oxidzing the nitric oxide therein to nitiogen tetroxide and recyclingsaid nitrogen tetroxide, separating said unreacted cycloalkane from saidliquid product and recycling it; and recovering said cycloalkyl nitritefrom the remainder of said product.

7. A process for the production of cycloalkyl nitrites which comprisesox*dizing ammonia so as to obtain an ammonia oxidation productcontaining water and nitric oxide, dehydrating the said ammoniaoxidation product to remove said water therefrom, oxidizing thedehydratd ammonia oxidation product to obtain a gaseous mixturecontaining nitrogen tetroxide, contaCting said gaseous mixture with aliquid cycloalkane thereby abmrbing said nitrogen tetroxide in saidcycloalkane to form a solution containing a minor proportion of nitrogentetroxide, introducing said solution having a temperature below about 40C.

into a reaction zone, maintaining said solution as it passes throughsaid reaction zone substantially in the liquid phase while heating it toa reaction temperature above about C. and maintaining it at saidtemperature while reacting said nitrogen tetroxide with part of saidcycloalkane 50 as to form a liquid product contamine unreaCtedcycloalkane and cyCloalkyl nitrite and a gascons product comprisingnitric oxide, separating said gaseous product from said liquid product,recycling said gaseous product to the stage in which the dehydratedammonia oxidation product is oxidized to form nitrogen tetroxide,separating said unreacted cycloalkane from said liquid product andrecycling it, and reCovering said cycloalkyl nitritefrcm the remainderof said product.

8. A process according to claim 7 in which an aqueous phase is formed inthe reaction zone, and is not permitted to separate from the liquidreaction product until the latter is removed from the reaction zone.

9. A continuous process for the production of cyciohexyl nitrite whichcomprises oxidizing ammonia with air so as to obtain an ammoniaoxidation product containing water and nitric oxide, dehydrating thesaid ammonia oxidation product to remove said water therefrom, oxidizingthe dehydrated ammonia oxidation product to form a gaseous mixturecontaining nitrogen tetroxide. absorbing said nitrogen oxide incyclohexane to form a solution containing about 5% to 15% of nitrogentetroxide, introducing said solution at a temperature below about 40 C.into a reaction zone, maintaining said solution as it passes throughsaid reaction. zone substantially in the liquid phase while heating itto a reaction temperature between about 100 C. and 250 C. andmaintaining it at said reaction temperature for a contact timesuflicient to react said nitrogen tetroxide with a part of saidcyclohexane so as to form a gaseous product comprising nitric oxide anda liquid product contalning cyclohexyl nitrite and unreactedcyclohexane, recycling said gaseous product to the stage in which thedehydrated ammonia oxidation product is oxidizgd to form nitrogentetroxide, separating said unreacted cyclohexane tram said liquidproduct, recycling said unreacted cyclohexane to the absorption stop.and recovering said cyclohexyl nitrite from the remainder of said liquidproduct.

10. A process for the production of nitrous acid esters of cycloalkaneswhich comprises dissolving a minor proportion of dry nitrogen tetroxidein said liquid cycloalkane and continuously passing the resultingsolution through an elongated reaction zone at an elevated teniperaturebetween about 100 C. and 250 C. and a pressure suiflcient to maintainthe hydrocarbon substantially in the liquid phase, at a rate suflicientto consume substantiaily all of the nitrogen tetroxide prior to leavingthe reactor.

11. A process according to claim 1 in which the cycloalkane iscyclohexane.

12. A process according to claim 1 in cycloalkane is cyclopentane.

13. A process according to claim 4 in which the cycloalkane iscycl0hexane.

14. A process according to claim 4 in which the cycloalkane iscyclopentane.

THOMAS F. DOUMANI. CLARENCE S. 00152.

EDWARD c. AI1AN, JR.

which the (Retercnces on following page) 17 be N Dat Num 1' ame eREFRNCES CITE 2,332,491 Senkus Oct. 19, 1943 The following reerences areof record in the 2,343,534 Cavanaugh et a]. Mar. 7, 1944 file of thispatient: 2,402,315 Crowder June 18, 1946 UNITED STATES PATENTS 5 OTHERREFERENCES Number lame Date Namztkin, Ber. dent. Chem.," vol. 42 (1909),

2,166,698 Alle July 18. 1939 pages 1372 t0,1376.

2,206,813 Hass et al July 2. 19 Schaarschmdt, "Zeit. Angewandt. Chem.."vol. 2,228,261 Elllnboe Jan. 14, 1941 10 37 (1924), pages 933-938.

2,240,558 Ellis May 8, 1941 Schorigln et al., "Ber. dent. Chem., vol. 672,285,601 McAllister June 9, 1942 (1934), pages 1362 to 1368.

2,298,387 K en on et al. Oct. 13, 1942 Kunter et 8.1., J. Chem. Soc."(London), 1936,. 2,314,615 Fra et al. Mar. 23, 1943 pages 285-286.

2,325,066 Marcotte July 27. 1943 15 Urbonski et 111., Comptes RendusAcad. Scl.,"

2,325,391 George et a]. July 27, 1943 vol, 204 (1937), pages 870-871.

